There are various systems for mobile phones, for instance, EGSM (extended global system for mobile communications) and DCS (digital cellular system) widely used mostly in Europe, GSM 850 (global system for mobile communications 850) and GSM 1900 (global system for mobile communications 1900) widely used in the U.S., and PDC (personal digital cellular system) used in Japan. According to recent rapid expansion of mobile phones, however, a frequency band allocated to each system cannot allow all users to use their mobile phones in major cities in advanced countries, resulting in difficulty in connection and thus causing such a problem that mobile phones are sometimes disconnected during communication. Thus, proposal was made to permit users to utilize a plurality of systems, thereby increasing substantially usable frequencies, and further to expand serviceable territories and to effectively use communications infrastructure of each system.
To utilize a plurality of systems, a user should conventionally have a mobile phone capable of communicating in plural systems. As a high-frequency part used in such a mobile phone, the inventors proposed a high-frequency switch module for switching transmitting circuits and receiving circuits in different communication systems (WO 00/55983).
The high-frequency switch module of WO 00/55983 comprises first and second filter circuits having different passbands, a switch circuit connected to the first filter circuit for switching a transmitting circuit and a receiving circuit of a communication system A, and a switch circuit connected to the second filter circuit for switching transmitting circuits of communication systems B, C, a receiving circuit of a communication system B and a receiving circuit of a communication system C.
The first and second filter circuits function as circuits for branching a received signal of the communication system A and received signals of the communication systems B, C. The switch circuit is a diode switch comprising a diode and a transmission line as main elements, and any one of pluralities of communication systems A, B, C is selected by controlling the diode in an ON or OFF state by applying voltage from a control circuit, thereby switching the antenna and transmitting circuits and receiving circuits of the communication systems A, B, C.
Specific examples of the communication systems A, B, C disclosed by WO 00/55983 are GSM, DCS 1800 and PCS, respectively. GSM corresponds to the EGSM, DCS 1800 corresponds to the GSM 1800, and PCS corresponds to the GSM 1900. Table 1 shows transmitting frequency and receiving frequency of each communication system.
TABLE 1CommunicationTransmittingReceiving FrequencySystemFrequency (MHz)(MHz)EGSM880 to 915925 to 960GSM 18001710 to 17851805 to 1880GSM 19001850 to 19101930 to 1990
JP 2000-165288 A, JP 2001-44885 A and JP 2002-171195 A disclose high-frequency composite parts used in pluralities of different communication systems.
With respect to GSM 1800 and GSM 1900 among communication systems handled by such high-frequency composite parts, it is appreciated that the transmitting frequency of GSM 1900 overlaps the receiving frequency of GSM 1800 in a range of 1850 MHz to 1880 MHz.
Problems in a case where the receiving frequency of the first communication system (GSM 1800) partially overlaps the transmitting frequency of the second communication system (GSM 1900) in the conventional high-frequency switch module for handling GSM 1800 and GSM 1900 described in WO 00/55983 will be explained using the equivalent circuit shown in FIG. 21.
This high-frequency switch module selects a transmitting mode of GSM 1800/GSM 1900, a receiving mode of GSM 1800, and a receiving mode of GSM 1900, by controlling voltage applied from control terminals as shown in Table 2.
TABLE 2ModeVC2VC3GSM 1800 TXV+0(Transmitting)GSM 1900 TXV+0(Transmitting)GSM 1800 RX00(Receiving)GSM 1900 RX0V+(Receiving)(A) GSM 1800/GSM 1900 transmitting mode
In the transmitting mode of GSM 1800 or GSM 1900, positive voltage (V+) is applied to a control terminal VC2, and zero voltage is applied to a control terminal VC3, to control diodes DD1, DD2 in an ON state. A transmission line ld3 has such proper length that its resonance frequency is in a frequency range (1710 MHz to 1910 MHz) of transmitting signals of GSM 1800 and GSM 1900, and grounded through the diode DD2 in an ON state and a capacitor cd4 for resonance. As a result, impedance is large (ideally infinitive) when the receiving circuits of GSM 1800 and GSM 1900 are viewed from the connection point IP2. Accordingly, transmitting signals sent from the transmitting circuit of GSM 1800 and GSM 1900 are sent to an antenna via the second filter circuit without leaking to the receiving circuit. At this time, the diodes DP1, DP2 are controlled in an OFF state.
However, because impedance is practically not sufficiently large in other frequencies than the resonance frequency when the receiving circuits of GSM 1800 and GSM 1900 are viewed from a connection point IP2, part of the transmitting signals of GSM 1800 and GSM 1900 (hereinafter referred to as “leak signals”) leak to the receiving circuits of GSM 1800 and GSM 1900 via the transmission line ld3. In addition, the resonance frequency may be changed by the unevenness of the capacitance of the capacitor cd4 and a capacitance component parasitic to the transmission line ld3, resulting in further increase in a signal leaking to the receiving circuits of GSM 1800 and GSM 1900.
Specifically, because the diodes DP1, DP2 are in an OFF state in the case of transmitting mode in GSM 1800, the leak signal does not appear in the receiving circuit of GSM 1900 by isolation when the diode DP1 is in an OFF state. On the other hand, the leak signal appearing in the receiving circuit of GSM 1800 via the transmission line lp2 is removed by a filter circuit (not shown) disposed upstream of the receiving circuit, resulting in substantially no leakage to the receiving circuit of GSM 1800. In the case of transmitting mode in GSM 1900, however, a leak signal of 1850 MHz to 1880 MHz overlapping the receiving frequency of GSM 1800 among those in a transmitting frequency of GSM 1900 appearing in the receiving circuit of GSM 1800 is supplied to the receiving circuit of GSM 1800 without being removed by the filter circuit, entering into analog-processing ICs constituting an LNA (low-noise amplifier)and a mixer in the receiving circuit and a modulator/demodulator, and thus causing the malfunction of these circuit parts.
(B) GSM 1800 Receiving Mode
In a receiving mode in GSM 1800, the diodes DP1, DP2, DD1 and DD2 are controlled in an OFF state by applying zero voltage to the control terminals VC2 and VC3. With the diode DD1 in an OFF state, impedance is large between the connection point IP2 and the transmitting circuit of GSM 1800/GSM 1900. With the diode DP1 in an OFF state, impedance is large between the connection point IP3 and the receiving circuit of GSM 1900. The connection point IP2 is thus connected to the receiving circuit of GSM 1800 via transmission lines ld3 and lp2.
(C) GSM 1900 Receiving Mode
In the receiving mode in GSM 1900, positive voltage is applied to the control terminal VC3, and zero voltage is applied to the control terminal VC2, to control the diodes DD1, DD2 in an OFF state and the diodes DP1, DP2 in an ON state. With the diode DD1 in an OFF state, impedance is large between the connection point IP2 and the transmitting circuit of GSM 1800/GSM 1900. The transmission line lp2 has such length that it is resonated at 1930 MHz to 1990 MHz in a frequency range of the received signal of GSM 1900. Accordingly, it is grounded through the diode DP2 in an ON state and the capacitor CP1 for resonance, resulting in large impedance when the receiving circuit of GSM 1800 is viewed from the connection point IP3. The connection point IP2 is thus connected to the receiving circuit of GSM 1900.
The diodes DP1, DP2 for switching the receiving circuits of GSM 1800 and GSM 1900 usually have low power consumption and small insertion loss. Such diodes are generally more likely subjected to distortion than diodes with large power consumption in an OFF state. Accordingly, the diodes DP1, DP2 in an OFF state are likely to distort the transmitting signals of GSM 1800, GSM 1900 leaking via the transmission line ld3, thereby generating harmonics having frequencies corresponding to integral multiples of those of these transmitting signals. These harmonics, which are added to the transmitting signals of GSM 1800, GSM 1900, are radiated from the antenna. With respect to such problems, none of WO 00/55983 JP 2000-165288 A, JP 2001-44885 A and JP 2002-171195 A provides any solutions.